Biggest mystery: What was the early environment on Mars like? We have a lot of information to indicate an early wet and warmer environment on Mars. We still don’t know how wet and how warm and how long such conditions lasted. A Mars that was warm and wet for short intervals is very different from one that was warm and wet for many millions or even tens or hundreds of millions of years.

Human landing: No idea.

Kim Stanley Robinson, author of The Mars Trilogy and 2312.

Memorable Moment: One night I went to the Planetary Society’s headquarters in Pasadena, California, for a Ray Bradbury birthday party, and after we celebrated the great Martian, who was charming and inspirational as always, a lot of us drove up to Mt. Wilson, the old observatory in the mountains overlooking Los Angeles. We had to climb stairs to get up to the platform where one looked through the giant eyepiece coming out of the side of the telescope. The astronomers on hand said viewing conditions for Mars were poor, though they were getting great views of Neptune’s moon Triton. But they aimed the beast at Mars for our sakes, and when I looked through the eyepiece, as big as a cereal bowl, and caught sight of Mars, I was stunned to see it filled the whole viewing circle, and was floating there, red with dark patches that, yes, were connected by straight black lines! I had to laugh.

Biggest surprise: I remain surprised at how big Olympus Mons is, like a round Colorado some 21 kilometers [13 miles] higher than the surrounding terrain. The whole volcano has a circular escarpment, a cliff some 10 kilometers high all the way around it. I set my climbing story “Green Mars” on one point of this cliff, and I am still curious to know what the explanation for that escarpment is.

Memorable moment: Looking at images of the south polar region of Mars, taken just as the sun was rising at latitudes equivalent to the Antarctic circle on Earth, when we were trying to find a safe landing site for Mars Polar Lander. [The spacecraft, designed to study the soil and climate near the south pole, crashed into the surface of Mars in 1999.] The pictures showed an ice-covered surface with unearthly terrain, with unexpected spidery channels and cracks—not a safe, smooth spot to set down a small lander.

Biggest mystery: The way that Mars’ seasonal dry ice polar caps re-work and erode the surface today. With the HiRISE camera on the orbiter, we have been able to study the spring evaporation of the planet’s seasonal dry ice polar caps. [MRO, launched in August 2005, is searching for evidence that water flowed on the surface of Mars for a long time.] We think that gas trapped below the ice causes the ice to break and allow jets to erupt, but we really don’t know for sure. We have a lot of circumstantial evidence, but no plumes have actually been detected. What we do know is that spring on Mars is an active process reshaping the surface at high latitudes in today’s climate.

Human landing: Before the end of this century. I hope they are able to go to the polar regions and see the spring jets.